Thermal isolation screen for isolating an electromagnetic inductor, and heat treatment installation comprising such a screen

ABSTRACT

Thermal insulation screen transparent to magnetic flux, intended to isolate an electromagnetic inductor with a transverse or pseudo-transverse field, from the radiation of a heated product ( 1 ), the thermal screen being made up of a matrix of blocks ( 7 ) made of a thermally insulating material and of a plurality of tubes ( 8 ) cooled by the circulation of a fluid, these tubes being imprisoned in said matrix of blocks and the tubes and the blocks being held in place by a support. The screen includes, behind the blocks, heat-conducting means ( 10 ) placed so as to intercept the heat flux passing through the blocks, these conducting means being thermally coupled to the cooled tubes ( 8 ) for discharging the heat flux into the tubes.

The invention relates to a cooled thermal insulation screen transparentto magnetic flux, intended to isolate an electromagnetic inductor havinga transverse or pseudo-transverse field from the radiation of a heatedproduct. The thermal screen is made up of a matrix of blocks made of athermally insulating material and of a plurality of metal tubes cooledby the circulation of a fluid, these tubes being imprisoned in saidmatrix of blocks, and the tubes and the matrix of blocks being held inplace on a support, which is located on the opposite side from theproduct to be heated.

The invention relates more particularly, but not exclusively, to athermal insulation screen for an electromagnetic induction device forheating a continuously running metal strip.

EP 1 349 431 discloses a chamber with a thermal insulation screen and asupport consisting of a sheath.

The materials of the support are electrically insulating and consist,for example, of a composite, glass fibres plus epoxy or other resin,glass or ceramic. The support for the thermal screen must not be exposedto excessively high temperatures, typically below 120° C. and preferablybelow 90° C., depending on the materials used for this support.

The electromagnetic coils for the induction heating are located on theopposite side of the screen from the product to be heated. To obtaingood energy efficiency, it is necessary for these coils to be as closeas possible to the product to be heated. It is therefore desirable forthe thickness of the various walls of the screen to be as small aspossible, while still ensuring sufficient thermal protection.

The tendency is to increase the temperature of the heated product,especially for metallurgical reasons, such that the risks of raising thetemperature of the support for the screen above the acceptable limitincrease substantially.

For better protection of the screen support, it may be conceivable toreduce the separation between the tubes cooled by the circulation of afluid. However, these tubes are preferably formed by straight segmentsof coils obtained by bending several times, in opposite directions, alength of tube. The diameter of the tubes used imposes a minimum radiusof curvature for bending them, so that it is not possible to reduce theseparation between the tubes below a value imposed by acceptable bendingof the tubes.

The drawbacks of the prior techniques for a thermal screen stem not onlyfrom the technological problems of manufacturing hairpins from tubesbent through 180° with smaller bending radii, but also thermal problemsinduced by the reduction in the centre-to-centre spacing of the tubes.Such a reduction in centre-to-centre spacing of the tubes results in:

-   -   increased thermal losses, as additional losses occur in the        magnetic field since the length of tubes lying in the field is        increased; and    -   lower surface temperature of the blocks, which may affect the        heating of the product.

The object of the invention is, above all, to provide a cooled thermalinsulation screen that allows better thermal protection of the supportfor the screen without substantially increasing the thickness of thelayer of insulating material, and without having to modify the spacingbetween the tubes cooled by the circulation of a fluid.

According to the invention, a thermal insulation screen transparent tomagnetic flux, intended to isolate an electromagnetic inductor with atransverse or pseudo-transverse field from the radiation of a heatedproduct, the thermal screen being made up of a matrix of blocks made ofa thermally insulating material and of a plurality of metal tubes cooledby the circulation of a fluid, these tubes being imprisoned in saidmatrix of blocks, the tubes and the blocks being held in place by asupport, is characterized in that heat-conducting means are provided tothe rear of the blocks, these conducting means being thermally coupledto the cooled tubes, said heat-conducting means intercepting the heatflux that passes through the blocks and discharging it into the cooledtubes.

The thermal screen according to the invention protects the support byreducing the operating temperature thereof.

Preferably, the heat-conducting means are also equipped with means forpreventing, or reducing, the thermal losses resulting from the inducedcurrents generated by the magnetic flux.

The heat-conducting means may be made of stainless steel, bronze, copperor another suitable material.

The heat-conducting means may be formed by a screen made of aheat-conducting material, especially a metallic material, thermallycoupled to the cooled tubes. Advantageously, the screen is formed bymetal fins that extend behind the blocks, each fin being in thermalcontact with a cooled tube.

The means for preventing, or reducing, the formation of induced currentsmay comprise slots made in the conducting means. The slots are mainlyoriented transversely to the longitudinal direction of the tubes, butthey may also be inclined thereto.

Advantageously, the slots open onto the longitudinal edges of the metalscreen and stop near the central portion.

The thermal screen is optimized when the induced currents are extremelysmall. To do this, fins having a sufficiently small area perpendicularto the magnetic field are provided. A good result is obtained byreducing the pitch of the slots, by varying the width of the slot and/orby varying the width of the fin and its thickness.

According to one embodiment, the fins may be produced by lengths of wirethat are oriented radially and fixed to the cooled tubes.

Each fin may extend, on each side of a tube, over substantially half thegap between two adjacent tubes.

Each fin may be welded to a corresponding tube. The region of the fin incontact with the tube may have the form of a cradle that follows aportion of the contour of the tube.

The fins may be formed by slots made in an extruded or drawn metalscreen/tube assembly made as a single part.

Preferably, the fins extend over the entire length of the tubes in theregion of the inductor to be protected from the heat flux.

The invention also relates to a heat treatment installation, especiallya furnace, which includes an electromagnetic inductor having atransverse or pseudo-transverse field, for heating a product,characterized in that it includes, for protecting the inductor againstthe thermal radiation of the heated product, at least one thermal screenas defined above.

The invention consists, apart from the arrangements presented above, ofa number of other arrangements that will be explained in greater detaillater with regard to exemplary embodiments described with reference tothe appended drawings, which are in no way limiting. In these drawings:

FIG. 1 is a diagram of a metal strip heat treatment installation with athermal insulation screen transparent to magnetic flux according to theinvention;

FIG. 2 is a cross section on a larger scale of the thermal insulationscreen, with cut-away portions;

FIG. 3 is a plan view with cut-away portions of the thermal insulationscreen;

FIG. 4 is a cross section on a larger scale of detail IV of FIG. 1showing the heat-conducting means according to the invention;

FIG. 5 is an end view of a heat-conducting means;

FIG. 6 is a partial plan view relating to FIG. 5; and

FIG. 7 illustrates, in vertical section, an alternative embodiment of aheat-conducting means.

FIG. 1 of the drawings shows schematically a vertical furnace H of aline for the heat treatment of a metal strip 1, especially a steelstrip, which runs continuously at a constant or variable speed. In theexample shown, the strip 1 runs vertically between two rollers, a lowerroller 2 and an upper roller 3 respectively. This example is notlimiting—the strip 1 could run horizontally in a horizontal furnace.

The furnace has generally a prismatic shape of rectangular cross sectionand its internal volume is isolated from the external atmosphere. Thecomposition of the gas mixture inside the furnace may vary depending onthe zones in question. This gas mixture comprises a relatively highproportion of hydrogen, favourable for heat exchange.

The furnace H includes at least one heating section 4 equipped, on eachside of the strip 1, with a half-inductor having a transverse orpseudo-transverse flux 5 a, 5 b. A pair of half-inductors constitutes aheating inductor.

A thermal insulation screen E transparent to the magnetic flux isprovided, at the half-inductors 5 a, 5 b, so as to thermally protect theinductors facing the heated product formed by the strip 1.

As may be seen in FIG. 2, the screen E comprises a support 6 formed froman electrically insulating and especially gas-impermeable material.Advantageously, the support 6 is made of a composite with an epoxy orsimilar resin.

The minimum dimension k (FIG. 2) of the space between opposed faces ofthe screen E, along the direction orthogonal to the strip 1, is imposedby considerations for the purpose of avoiding any contact between thestrip and the internal surface of the screen. This dimension k may bearound twenty centimetres.

The internal faces of the support 6 support and hold in place, byfastening means (not shown), a matrix B of blocks 7 made of thermallyinsulating material, especially ceramic, and a plurality of tubes 8cooled by the circulation of a fluid, generally water. The tubes 8 aregenerally made of metal, but they could be made of another material, forexample glass completely transparent to the magnetic field. The tubes 8are imprisoned in the matrix of blocks 7 and protected against thedirect radiation from the strip 1.

The tubes 8 preferably consist of straight segments. According to theexample shown in FIG. 3, the tubes 8 are segments of a continuous coilobtained by successively bending a tube through 180° in oppositedirections. An inlet and an outlet for the cooling fluid are providedfor each coil. Several coils constituting successive panels may bejuxtaposed against the internal wall of the support 6. The internaldiameter of the tubes 8 is about eight to ten millimetres.

The blocks 7, as may be seen in FIG. 2 and 4, are formed bysubstantially rectangular parallelepipedal blocks, the two longitudinalfaces of which include recesses 7 a, 7 b in the form of cylindricalportions suitable for following the contour of two successive tubes 8.The blocks 7 may thus be engaged between two straight tubes 8. Theblocks 7 bear against one another along the longitudinal direction ofthe tubes 8. A small transverse space 9 (FIG. 4) may be left between twoadjacent blocks 7.

The front face 7 c of the blocks 7 is the one directly exposed to thethermal radiation from the strip 1. The rear face 7 d of the blocks,which is the face furthest from the strip 1, is at a temperatureconsiderably below that of the face 7 c because of the thermalresistance presented by the block 7 and because of the heat extracted bythe tubes 8 and the cooling fluid.

In order for the strip 1 to be heated with maximum energy efficiency byelectromagnetic induction with a transverse or pseudo-transverse flux,it is desirable to place the inductors as close as possible to the strip1. When the treatment temperature of the strip 1 increases, there is arisk of the support 6 being exposed to too high a temperature.

To ensure effective thermal protection of the support 6, without thethickness of the insulating blocks 7 being increased and withoutreducing the distance between successive tubes 8, the inventionprovides, at the rear face 7 d of the blocks, heat-conducting means 10placed so as to intercept the heat flux Q passing through the blocks 7.These conducting means 10 are thermally coupled to the cooled tubes 8 inorder to discharge the heat flux Q into the tubes and into the coolingliquid circulating in the tubes. Furthermore, the conducting means 10are equipped, as illustrated in FIG. 6, with means 11 for preventing orreducing the formation of induced currents generated by the transversealternating electromagnetic flux.

Preferably, the conducting means 10 consist of a metal screen S (FIG. 4)of small thickness j especially less than five millimetres. The screen Scovers most of the rear faces 7 d of the blocks.

The metal screen S is formed by a set of metal fins 12 separated fromone another. Associated with each tube 8 are metal fins 12 covering therear face 7 d of the blocks, each fin being in thermal contact with theassociated cooled tube 8. The terms “heat sink” or “heat extractor”could be used as synonyms for the term “fin”.

Each fin 12 is formed by a metal strip preferably extending over theentire length of the tube 8. The strip has a projecting centrallongitudinal portion 12 a engaged in a free space 13 between the rearedges of two successive blocks 7. The central portion 12 a comes intocontact with the wall of the tube 8 to which it is preferably welded.The metal strip extends on either side of the central portion 12 a,symmetrically or asymmetrically, with blades 12 b, 12 c meetingsubstantially at mid-width of each block 7. Such an arrangement makes itpossible to limit the length of the path for discharging the heat intothe tube 8 to substantially one half of the width of a block 7. The endsof the blades 12 b, 12 c of two successive fins are separated by a gapof small width m (FIG. 4), especially less than five millimetres, andpreferably less than two millimetres. The rear face 7 d of each block isthus practically entirely covered by fins 12.

In the example shown in FIG. 2, the cross section of the thermal screenhas a substantially rectangular closed outline. This example is notlimiting. The thermal screen may consist, on each side of the strip, ofa plate having several layers (insulating blocks, cooled tubes,conducting means). In this case, the cross section of the screen wouldbe a straight segment, as shown in FIG. 4. A plate-shaped heat screenaccording to the invention is particularly suitable for installationssuch as those intended in Patent Applications FR 05/06462 and FR05/06463 filed on 24 Jun. 2005 by the same filing company CELES.

According to the exemplary embodiment shown in FIG. 4, the centralportion 12 a of the fin has the shape of a dome with its convex sideturned towards the tube 8, said dome having oblong openings 14 forwelding it.

As a variant, as illustrated in FIG. 7, the projecting central portion12 a has a concavity forming a cradle for following the facing portionof the tube 8 and providing a larger contact area for heat transfer.

According to another embodiment, the fins may be produced by slots madein a tube 8/metal screen S assembly extruded or drawn as a single part.

The means 11 for preventing, or reducing, the formation of inducedcurrents generated by the magnetic flux preferably comprise slots 15made in the fins 12. The slots are oriented mainly transversely to thelongitudinal direction of the fins, as illustrated in FIG. 6, but theymay also be inclined thereto. The slots 15, which are parallel to oneanother, open onto the longitudinal edges of the metal screen S and areclosed near the central portion 12 a.

The thermal screen is optimized when the induced currents are extremelysmall. To do this, fins having a sufficiently small area perpendicularto the magnetic field are provided. A good result is obtained byreducing the pitch of the slots, by varying the width of the slot,especially by varying the thickness of a saw cut constituting the slot,and/or by varying the width of the fin and the thickness. According toone example, the slot width is 0.5 mm and the fin width between twoslots is adapted to the electrical loss of the currents induced in thisfin.

As a variant, the fins may be produced by lengths of wire. The lengthsof wire are oriented transversely, radially or obliquely to the coolingtubes 8 and are fixed, especially welded, to the tubes 8, the assemblyresembling a comb, the teeth of which would be formed by the lengths ofwire. The diameter of the wire may be 1 mm with a gap between each wireof 0.3 mm, this gap constituting the slot 15.

The fin 12, seen from above as illustrated in FIG. 6, has a shape thatresembles that of a fishbone.

The presence of the slots 15, which greatly reduce the induced currentsgenerated by the magnetic flux, reduces parasitic heating that wouldlower the energy efficiency.

Advantageously, the tubes 8 and/or the fins 12 are made of stainlesssteel. As a variant, the tubes 8 and/or the fins 12 are made of bronzeor copper, while other metals may also be envisaged.

The operation results from the foregoing explanations.

The transverse alternating magnetic field heats the strip 1, whichradiates heat towards the surfaces that surround this strip. The heatflux Q that passes through the blocks 7 to reach the rear face 7 d ispredominantly intercepted by the fins 12 and discharged into the tubes 8and into the cooling fluid. The support 6 is better protected againstthe heating that would be due to this flux Q.

The invention makes it possible, without the thickness of the heatscreen E being substantially increased and without the spacing of thetubes 8 being modified, to provide effective thermal protection of thesupport 6 for high temperatures of the strip 1, for example 1150° C.

Although the description has been given with regard to a strip 1, theinvention applies to other types of metal products, especially steel,copper or aluminium wires or plates. The product may or may not be arunning product.

1. Thermal insulation screen transparent to magnetic flux, intended to isolate an electromagnetic inductor with a transverse or pseudo-transverse field from the radiation of a heated product, the thermal screen being made up of a matrix of blocks made of a thermally insulating material and of a plurality of tubes cooled by the circulation of a fluid, these tubes being imprisoned in said matrix of blocks, the tubes and the blocks being held in place by a support, characterized in that it includes, behind the blocks, heat-conducting means placed so as to intercept the heat flux passing through the blocks, these conducting means being thermally coupled to the cooled tubes for discharging the heat flux into the tubes.
 2. Thermal insulation screen according to claim 1, wherein the heat-conducting means are equipped with means for preventing, or reducing, the formation of currents induced by the magnetic flux.
 3. Thermal insulation screen according to claim 1, wherein the heat-conducting means are formed by a metal screen thermally coupled to the cooled tubes.
 4. Thermal insulation screen according to claim 3, wherein the metal screen is formed by metal fins that extend to the rear of the blocks, each fin being in thermal contact with a cooled tube.
 5. Thermal insulation screen according to claim 4, wherein the fins are produced by lengths of metal wire that are fixed to the cooling tubes.
 6. Thermal insulation screen according to claim 1, wherein the means for preventing, or reducing, the formation of induced currents generated by the magnetic flux comprise slots made in the conducting means.
 7. Thermal insulation screen according to claim 6, wherein the slots are oriented transversely to the longitudinal direction of the tubes.
 8. Thermal insulation screen according to the claim 4, wherein the slots open onto the longitudinal edges of the metal screen and are closed near the central portion.
 9. Thermal insulation screen according to claim 4, wherein each fin extends on either side of a tube.
 10. Thermal insulation screen according to claim 9, wherein each fin extends, on each side of a tube, over substantially half the gap between two adjacent tubes.
 11. Thermal insulation screen according to claim 4, wherein each fin is welded to a tube.
 12. Thermal insulation screen according to claim 4, wherein the fins are formed by slots made in an extruded or drawn metal screen/tube assembly made as a single part.
 13. Thermal insulation screen according to claim 4, wherein the region of the fin adjacent to the tube has the form of a cradle that follows a portion of the contour of the tube.
 14. Thermal insulation screen according to claim 4, wherein the fins are provided over the entire length of the tubes, in the region of the inductor to be protected from the heat flux.
 15. Heat treatment installation, especially a furnace, which includes an electromagnetic inductor having a transverse or pseudo-transverse field, for heating a product, characterized in that it includes, for protecting the inductor against the thermal radiation of the heated product, at least one thermal screen according to claim
 1. 